Contemporary combustion engines beneficially operate with combustion in their one or more combustion chambers occurring at a high combustion temperature for achieving more efficient engine operation. However, engine operation at high combustion temperatures has associated therewith generation of soot and nitrogen oxide exhaust products, for example nitrous oxides (NOx). Soot and nitrogen oxide exhaust products are hazardous materials when ejected into the environment. For reducing concentrations of exhaust pollutants to within limits defined by legislation, for example contemporary European legislation pertaining to exhaust emissions, vehicle manufacturers conventionally employ Selective Catalytic Reduction (SCR) and/or Exhaust Gas Recirculation (EGR) in their vehicles. SCR is effective for absorbing nitrous oxides whereas EGR ensures more complete oxidation of nitrogen oxides present in exhaust gases.
SCR operation is improved by using chemical agents which are transported and/or stored on vehicles, marine facilities and industrial facilities. For example, “AdBlue” and “DEF” are trade names for a chemical agent corresponding substantially to aqueous urea solution 32.5%. This solution is injected into exhaust gases of modern diesel engines during a post-combustion process for treating engine exhaust gases for reducing a proportion of harmful nitrous oxide (NOx) present in these gases. AdBlue, DEF, “Adblu” is only ever used in conjunction with an SCR. Moreover, vehicles which are equipped with an SCR will carry an AdBlue, DEF or “Adblu” storage tank in addition to a fuel tank. In operation, AdBlue, DEF, or “Adblu” is transferred from the storage tank and injected under pressure into exhaust gases where a series of chemical reactions occur as outlined in Table 1 below.
In FIG. 1, an exhaust system for a combustion engine 15 is indicated generally by 10. The system 10 includes in sequence an inlet 20 for receiving combustion gases from the engine 15, an oxidation catalyst 30, an AdBlue injection region 40, an hydrolysis catalyst 50, an SCR catalyst 60, an oxidation catalyst 70 and finally an outlet 80.
TABLE 1RegionReactionOxidation catalyst 302NO + O2 = 2NO22CO + O2 = 2CO24HC + 3O2 = 2CO2 + 2H2OHydrolysis catalyst 50CO(NH2)2 + H2O = 2NH3 + CO2SCR catalyst 608NH3 + 6NO2 + 7N2 + 12H2O4NH3 + 4NO = O2 + 4N2 + 6H2O2NH3 + NO + NO2 = 2N2 + 3H2OOxidation catalyst 704NH3 + 3O2 = 2N2 + 6 H2O
Essentially, the process occurring in the exhaust system 10 involves an AdBlue mixture being passed onto a super-heated porous ceramic head of the SCR catalyst 60 whereat the de-ionised water evaporates and remaining urea of the injected AdBlue is passed on as a reagent which is operable to break nitrous oxide components (NOx) into mostly Nitrogen and water. Any contaminants present in the AdBlue will collect on the SCR catalyst 60, eventually causing it to clog and fail. It is therefore imperative that the AdBlue remains free from impurities through all stages of production, storage and dispensing.
Detection of ingredients in AdBlue, DEF or “Adblu” is highly desirable; detection of such ingredients is to be understood to be a quality measurement, not merely a measurement of urea concentration which is something different. Agricultural grades of urea are designated for use in agriculture, for example to improve soil quality, whereas industrial grades of urea, for example Adblue, DEF, are adapted for use in road vehicles, for example trucks and buses. When agricultural urea is cheaper on account of more contaminants therein in comparison to industrial AdBlue, there is a temptation for users to employ agricultural AdBlue in trucks and buses for exhaust gas treatment during road use in breach of legal regulations; impurities in agricultural grades of urea can potentially poison catalysts in exhaust systems as well as risk creating airborne pollution including, for example, heavy metals. Moreover, there is also a potential risk that agricultural urea or industrial AdBlue, DEF is contaminated by extraneous material on account of conditions of its storage, for example in unclean tanks which have previously employed to store other materials, for example insecticides.
Another risk is an unintentional confusion by personnel of different tanks available on a vehicle, wherein Diesel fuel, wind screen washer liquid and similar is filled into the Adblue, DEF tank by accident. Another risk is that personnel try to substitute Adblue, DEF with other materials, for example a saline solution, in Adblue, DEF tanks of vehicles to save money. Many farmers face extreme economic pressures which can result in them being attempted to save wherever feasible
Unwanted contamination in Adblue, DEF can have other consequences, for example resulting in a catastrophic event such as a complete SCR failure. Attempting to operate vehicles with defective SCR can represent a criminal offence. AdBlue, DEF is very susceptible to contamination from both foreign matter and incorrect material selection. A main influence concerns the de-ionised water element of the solution which draws ions from materials which it comes into contact with; this changes the chemical composition of the AdBlue, DEF and causes salts to form which in turn clog the ceramic head on the SCR catalyst 60. Most common causes of premature failure of the SCR catalyst 60 are typically either a result of the ingress of damaged pump parts being accidentally transferred into the AdBlue tank or as a result of incorrect material selection. Inert materials should thus always be used to handle AdBlue.
Thus, poor quality of AdBlue can cause increased vehicle pollution and also damage to engine exhaust gas systems. It is not always possible for vehicle drivers to be certain about the origin of AdBlue, DEF that is utilized in their vehicles, and can in consequence unintentionally cause, for example, dangerous pollution from contaminants present in the AdBlue, DEF. However, devices for metering concentrations of urea solution are known, for example a device as described in published United States patent application US 2005/0011183A1 (Ripper et al.). The device includes a sensor unit for monitoring one or more physical state variables of an enzyme-free urea solution. The sensor unit is designed for detecting pH, a dielectric constant of the urea solution and/or conduction of the enzyme-free urea solution. Electrodes of the sensor unit are, for example, implemented as an intermeshed comb-like structure. Moreover, the sensor unit includes a vibration generator for testing mechanical properties of the urea solution, wherein the vibration generator includes a quartz oscillator and/or a piezoelectric crystal. It is to be appreciated that the device described in published United States patent application US 2005/0011183A1 (Ripper et al.) is essentially a urea solution concentration measureing device, which is very different to a urea solution quality measuring apparatus.
However, such known devices for metering urea solution concentration are not sensitive enough and/or accurate enough for distinguishing between various qualities of urea solution for vehicle use, for example distinguishing agricultural-grade urea from transport-grade urea due to presence of trace metal salts that hardly cause any perceptible change in urea solution concentration. Urea solution quality spatially varies within a vehicle urea tank and known sensors measure in a given spatial locality which is potentially not representative of the generally quality of the urea solution in the tank; thus, the aforementioned device described in published United States patent application US 2005/0011183A1 (Ripper et al.) would be unsuitable for constructing a urea quality monitoring apparatus.